1. Field of the Invention
The present invention relates generally to digital telecommunication services and, more particularly, to interoperability between two mutually incompatible digital telecommunication protocols.
2. Description of the Prior Art
Within the United States and Canada there exists two mutually incompatible protocols for exchanging digital data between computers using digital telecommunications. These two mutually incompatible conventions are respectively called Integrated Services Digital Network ("ISDN") Basic Rate Access and Switched-56 Access. The incompatibility between these two systems is of such a character that a telephone device which communicates using the ISDN protocol cannot exchange data with a Switched-56 Access. Conversely, a telephone device that communicates using the Switched-56 protocol cannot exchange data with an ISDN Access. However, since the ISDN Basic Rate Access protocol is implemented more extensively throughout the world, e.g., Japan, Australia, Europe, etc., it would be advantageous if were possible for ISDN devices to transmit and receive digital data over a Switched-56 Access.
While most local and long-distance telephone carriers in the United States offers Switched-56 digital telecommunications service, some Local Exchange Carriers do not offer ISDN Access. Switched-56 Access provides a user with a dial-up digital telecommunications capability for transmitting and receiving digital data either at 56 kilobits per second ("kbps") in a synchronous telecommunication mode, or 57.6 kbps in an asynchronous telecommunication mode. Switched-56 digital telecommunications service is widely used for low-bandwidth video conferencing to send and receive acceptable sound and pictures using only two Switched-56 telephone lines, i.e., transmitting and receiving at 112 kbps. Consequently, at present every major video conferencing equipment manufacturer incorporates a Switched-56 capability in its equipment. Analogous equipment is available for ISDN telecommunications throughout those regions of the world in which ISDN services are available.
Physically, a Switched-56 Access in accordance with the American Telephone and Telegraph Company's ("AT&T's") standard consists of one or two twisted pairs of wires which carry time compressed multiplexed ("TCM") digital signals to provide one full-duplex synchronous transmission path at a data rate of 56 kbps. Conversely, in accordance with a recommendation of an International Telegraph and Telephone Consultative Committee ("CCITT"), an ISDN Basic Rate Access provides two (2) full-duplex 64 kbps digital data channels, called channel B1 and channel B2, plus another full-duplex 16 kbps digital channel, called a D-channel. Under the CCITT recommendation, using time division multiplexing, all three of these digital data channels may be transmitted over a single pair of twisted wires, or over two pairs of twisted wires. The unrestricted data rate at which digital data may be transmitted over twisted pairs of wires in accordance with the ISDN recommendation for Basic Rate Access is 144 kbps, i.e. 64 kbps on each of the B1 and B2 channels plus 16 kbps on the D-channel.
As described in an AT&T Technical Reference entitled "Special Access Connections To The AT&T Network," TR 41458, published April 1990, while a Switched-56 Access actually transmits and receives bidirectional TCM digital data at 128 kbps (64 kbps being alternatively first transmitted and then received), the eighth bit of each octet is not available for data transmission because it is subject to over-writing if data is transmitted between telephone company switches. Thus, with a Switched-56 Access user data may occupy only bits 1 through 7 of each octet thereby providing an effective data transfer rate of only 56 kbps.
Within North America, telephone network switches capable of ISDN Basic Rate Access telecommunications must also be capable of concurrent Switched-56 telecommunications. To accommodate such concurrent operation, the CCITT Recommendations Q.930-Q.940 for ISDN Basic Rate Access permits a restricted mode of operation for each of the ISDN digital data channels B1 and B2. In accordance with these CCITT Recommendations, if an ISDN device exchanges digital data with a Basic Rate Access operating in restricted mode, then the data channel B1 or B2 transmits and receives data at an effective data rate of 56 kbps rather than at the unrestricted rate of 64 kbps. If an ISDN device operates in this restricted mode, it discards the eighth bit of every octet (eight bits) received from the Basic Rate Access. Analogously, an ISDN device transmitting digital data into a restricted ISDN Basic Rate Access uses only the first seven bits of each successive octet, and does not place any of the data being transmitted into the eighth bit of each octet. During set-up of an ISDN telephone call, signals transmitted from the telephone company switch notify the ISDN terminal equipment that digital data is to be transferred in restricted mode, or the terminal equipment placing a telephone call may notify the telephone company switch of that fact.